1. Field of the Invention
This invention relates to an apparatus such as a memory apparatus for effecting super-high density information recording (erasing) and/or reproduction by the use, for example, of a tunnel current.
2. Related Background Art
Semiconductor memories and magnetic memories using a magnetic material or a semiconductor as a material have heretofore been the mainstream, but in recent years, with the advancement of the laser technique, inexpensive and highly dense recording mediums using optical memories using organic thin film of organic coloring matters, photopolymer or the like have made their advent.
On the other hand, recently, a scanning type tunnel microscope (hereinafter abbreviated as STM) through which the electron structure of the surface atom of a conductor can be directly observed has been developed [G. Binning et al., Phys. Rev. Lett., 49, 57 (1982)] and measurement of high resolving power of a real spatial image has become possible irrespective of single crystal or an amorphous material, and STM has the advantage that it can observe with low electric power without imparting any damage by a current to a sample and further, can operate even in the atmosphere and can be used for various materials and therefore, wide applications thereof are expected.
The STM utilizes the fact that when a voltage is applied between a metallic probe (probe electrode) and an electrically conductive substance and the probe is brought close to the substance to a distance on the order of 1 nm, a tunnel current flows. This current is very sensitive to any variation in the distance between the two. By scanning the probe so as to keep the tunnel current constant, various kinds of information regarding all electron clouds in a real spatial image can also be read. In this case, the resolving power in the in-plane direction is on the order of 0.1 nm.
Accordingly, if the principle of the STM is applied, it will be possible to effect high-density recording and reproduction sufficiently at the atom order (sub-namometer). For example, in the recording-reproducing apparatus disclosed in Japanese Laid-Open Patent Application No. 61-80536, atomic particles adhering to the surface of a medium are removed by an electron beam or the like to thereby effect writing, and this data is reproduced by an STM.
A method of effecting recording and reproduction in an STM by the use of a thin film layer of a material having a memory effect for the switching characteristic of a voltage or a current, for example, .pi. electron line organic compound or a chalcogen compound, as a recording layer is disclosed in Japanese Laid-Open Patent Application No. 63-161552, Japanese Laid-Open Patent Application No. 63-161553, etc. According to this method, if the bit size of recording is 10 nm, recording and reproduction of a capacity as great as 10.sup.12 bit/cm.sup.2 is possible.
To effect recording or reproduction on a memory medium in an actual apparatus, it is necessary to maintain and stabilize the spacing between the recording layer and the probe electrode at a distance over which a tunnel current flows. So, heretofore, the control of the spacing has been effected so that the average spacing between the recording layer and the probe electrode may assume a predetermined value, that is, the average value of the tunnel current may be a predetermined value. This is disclosed, for example, in Japanese Laid-Open Patent Application No. 1-133239. This method is such that servo control is applied so as to fix and maintain the probe electrode near the average position of vertical movement so that the probe electrode may not vertically move without omission in conformity with the value of the recording bit.
Also, when the tunnel current is to be intensified to write information onto the recording layer, the hold control of an actuator (hereinafter referred to as the pre-value hold) has been effected so as to hold the position in the Z direction at a predetermined value so that the spacing between the probe electrode and the recording layer may not be greatly varied by a great current then flowing.
However, the system which effects the control of the spacing between the probe electrode and the recording layer by the use of the average value of the tunnel current suffers from the following three problems:
(1) When the scanning speed of the probe electrode is increased, the tip end portion of the probe electrode may collide with the convex recording bit of the recording layer. Particularly when the shape of the recording bits is not uniform, the possibility of the collision occurring frequently becomes high.
What determines the spacing between the probe electrode and the recording layer is the average value of the tunnel currents flowing in the ON state and the OFF state, respectively. This average value is determined chiefly by the tunnel current when the recording layer is in the ON state. The ON state is the state of the convexly shaped bits in a case where recording is effected by the modulation of the surface shape, and is a state in which the electron density is high and the electrical conductivity with respect to the base electrode has become high in the case of the recording in the modulation of the electron state (the electrical conductivity state). Therefore, the spacing between the probe electrode and the recording layer is affected by the pattern of data rows written on the recording layer, i.e., the detection frequency of the ON state bits.
The tunnel current is controlled by the very minute spacing (usually 1 nm or less) between the probe electrode and the recording layer and therefore, even if use is made of a modulation system which does not have a direct current component such as EFM, any slight variation in the average tunnel current by the difference in substance between data being recorded will result in the collision with the probe electrode at the ON state bit.
Also, from the necessity of reading out data at a high speed, the cutoff frequency of a low-pass filter is set to a sufficiently low value as compared with the signal band frequency of recorded data. However, if the cutoff frequency of the low-pass filter is set to a low value, the probe electrode will not be able to be retracted at a high speed and may collide with the recording layer when a greatly convexly shaped ON state bit by the irregularity of writing during recording is detected.
When the probe electrode collides with the recording layer, not only recorded data are lost but also the probe electrode may be unrecoverably damaged, and the above-noted problem will remarkably reduce the reliability of an information reproducing apparatus utilizing a tunnel current.
(2) If during data writing, the spacing between the probe electrode and the recording layer is pre-value-held and controlled, there may occur irregularity in the shape of written bits. Particularly, in the first bit writing of data rows, the possibility of the contact of the probe electrode with the recording layer shape-modulated by a writing current occurring frequently is high.
The states of an example of the prior art during recording are shown in FIGS. 6, 7 and 8 of the accompanying drawings. FIG. 6 shows the state before the application of a writing pulse voltage, and FIG. 7 shows the state after the application of the writing pulse voltage, i.e., a state in which the surface of the recording layer is melted and expanded by energy poured from the pulse voltage into the recording layer and convexly shaped recording is accomplished rightly.
However, this recording method has a very unstable factor. When an electric current is caused to flow from the probe electrode to the recording layer to thereby melt the recording layer, the recording layer begins to expand. At this time, the spacing between the probe electrode and the recording layer becomes smaller by the amount of expansion of the recording layer. This reduction in the spacing increases the electric current flowing between the probe electrode and the recording layer. This increased electric current further expedites the melting of the recording layer to thereby further expand the recording layer toward the probe electrode. As a result, as shown in FIG. 8, the recording layer collides with the probe electrode and an extremely large recording bit is formed. The formation of such recording bits irregular in size not only reduces the S/N ratio of the signal, but also causes the collision of the probe electrode during the aforedescribed reproduction.
A similar problem has arisen when recording is effected with the electron state of the surface of the recording layer modulated. When the electron state of the surface of the recording layer is modulated, the melting and expansion of the recording layer do not occur, but recording is effected by the modulation of the electrical conductivity through the recording layer between the base electrode and the probe electrode.
However, if a writing pulse voltage is applied when the probe electrode is very close to the recording layer when the probe electrode is pre-value-held, the electron state of the recording layer changes and a state in which the electrical conductivity is high is formed and also, an excessive current exceeding the normal recording level flows to the base electrode. Thereupon, this excessive current heats the base electrode of the recording layer, whereby the base electrode is melted and expanded. Due to such damage of the base electrode, high density recording cannot be accomplished.
(3) As regards the average value control and the pre-value hold control of the spacing between the probe electrode and the recording layer, it is difficult to effect appropriate control for both of initial writing and re-writing.
This is because the control of the spacing between the probe electrode and the recording layer during data writing is determined by the pre-value of the average value of the tunnel current and therefore a proper spacing is not always obtained and there is the possibility of deviation from a proper value occurring. This deviation from the proper spacing is greatly varied by whether the state of the recording layer is initial writing or re-writing. Further, in the case of re-writing, deviation from the proper spacing is also caused by formerly recorded information (ON or OFF).